Resistive Barriers vs. Water Balance Covers: Tales of Two Cover Designs in

Seasonal Semi-Arid Climates

Craig H. Benson, PhD, PE, NAEUniversity of Virginia/CRESP

chbenson@virginia.edu

Cover Strategy:Resistive Barrier vs. Water Balance Covers

Conventional Cover Water Balance Cover

Surface

Layer

Interim

Clay

Barrier

GeomembraneStorage

Layer

Interim

Capillary Break

Clay RadonBarrier

GravelAmended

Water Storage

& Protection

BiotaBarrier

Sand

200 mm

900 mm

300 mm

300 mm

300 mm

Tailings

300 mm

150 mm

450 mm

450 mm

Clay RadonBarrier

FrostProtection

Bedding

Riprap

Tailings

Sand

Clay RadonBarrier

GravelAmended

Water Storage

& Protection

200 mm

1000 mm

910 mm

Tailings

ClayRadonBarrier

Interim

Monticello Disposal Facility

White MesaMill Tailings Facility

CheneyDisposal Facility

Cheney DisposalFacility

Monticello DisposalFacility

White MesaMill

Utah Colorado

Validate Performance with ACAP LysimeterACAP lysimeters used to confirm a design meets performance goal by directly monitoring percolation and to understand, when necessary,

the hydrologic processes controlling percolation.

Cover

Soils

Instrument

NestDiversion

Berm

Geomembrane

Liner

Geocomposite

DrainInterim

Cover

Soil

Root

Barrier

Surface

Runoff

Percolation

Diversion

Berm

Benson,C.,Albright,W.,Fratta,D.,Tinjum,J.,Kucukkirca,E.,Lee,S.,Scalia,J.,Schlicht,P.,Wang,X.(2011),EngineeredCoversforWasteContainment:ChangesinEngineeringProperties&ImplicationsforLong-TermPerformanceAssessment,NUREG/CR-7028,OfficeofResearch,U.S.NuclearRegulatory

Commission,Washington.

ResistiveWater Balance

Blackfoot Bridge, August 2013

Formwork

Installing GCLL

Base Geomembrane

Filling Alluvium

GCLL

July 2015

Elevation: 1500 mmPrecipitation: 230 mmClimate: Seasonal semi-aridCover Thickness: 1.4 m

Armored Cover at Cheney Disposal Facility

300 mm

150 mm

450 mm

450 mm

Clay RadonBarrier

FrostProtection

Bedding

Riprap

Tailings

Cheney Water Balance Record

0

500

1000

1500

2000

2500

0

20

40

60

80

100

10/31/07 6/12/09 1/24/11 9/6/12 4/20/14 12/2/15 7/15/17

Cu

mula

tive

Pre

cip

ita

tion

an

d E

vap

otr

asn

pir

ation

, a

nd S

oil W

ate

r S

tora

ge

(m

m)

Cu

mula

tive

Ru

noff a

nd P

erc

ola

tion

(mm

)

Soil Water Storage

Surface Runoff

Perc

On-Site Precipitation

On-Site ET

NWS ET

NWS Precipitation

• Most precipitation becomes ET

• Perc much higher later in record.

• Subtle variation in soil water storage.

• Very little runoff.

Cheney Water Content Record

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

Jan

/1/0

8

Jan

/1/1

0

Jan

/1/1

2

Jan

/1/1

4

Jan

/1/1

6

Bedding

Frost Protect

Radon Upper

Radon Lower

Sand

Vo

lum

etr

ic W

ate

r C

onte

nt (-

)

• Bedding and frost protection layer vary seasonally

• Radon barrier saturated at end of record

• Increase in water content of sand consistent with percolation

Cheney Annual Water Balance

• ET largest fraction, but usually < precip

• Perc relatively low, except during wetter years.

• Perc tied directly to integrity of clay radon barrier.

0

200

400

600

800

1000

0

20

40

60

80

100

20

08

20

09

2010

2011

20

12

20

13

2014

2015

20

16

2017

Evapotranspiration

Precipitation

Percolation

Pre

cip

ita

tion &

Evap

otr

an

sp

iratio

n (

mm

)

Pe

rcola

tion

(mm

)

Note: Y2 scale 10x < than Y1

Cheney: Percolation & Precipitation• Most of

precipitation becomes ET

• Perc much higher during wetter years

• Barrier integrity critical for managing wet conditions.

50

100

150

200

250

300

350

400

0

10

20

30

40

50

60

50 100 150 200 250 300 350 400

Evapotranspiration

Percolation

Eva

potr

anspira

tio

n (

mm

)

Perc

ola

tion (m

m)

Precipitation (mm)

Water Balance Cover at Monticello Disposal FacilityElevation: 2150 mmPrecipitation: 370 mmClimate: Seasonal semi-aridCover Thickness: 2.0 m

Clay RadonBarrier

GravelAmended

Water Storage

& Protection

BiotaBarrier

Sand

200 mm

900 mm

300 mm

300 mm

300 mm

Tailings

Monticello Water Balance Record

On-Site ET

0

2000

4000

6000

8000

0

100

200

300

400

500

600

8/1

/00

12

/12

/02

4/2

3/0

5

9/3

/07

1/1

3/1

0

5/2

5/1

2

10/5

/14

2/1

5/1

7

Cum

ula

tive

Pre

cip

ita

tion

an

d E

va

po

tra

nsp

ira

tio

n (

mm

)

Cu

mu

lativ

e P

erc

ola

tion

an

d S

urfa

ce

Ru

no

ff,a

nd

So

il Wa

ter S

tora

ge

(mm

)

NWS Precipitation

Surface Runoff

Percolation

Soil Water Storage

NWS ET

Monticello Water Balance Cover

On-Site Precipitation

• Nearly all precip becomes ET

• Storing & releasing water each year

• Runoff very small fraction (minimal erosion)

• Avg percolation rate < 0.5 mm/yr

Monticello Water Content Record

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

Jan

/1/0

0

Jan

/1/0

5

Ja

n/1

/10

Jan

/1/1

5

Topsoil

Lower Storage

Deep Storage

Sand

Vo

lum

etr

ic W

ate

r C

onte

nt (-

)

• High frequency variation near surface.

• Dampens with depth

• Runoff very small fraction (minimal erosion)

• Avg percolation rate < 0.5 mm/yr

Monticello Annual Water Balance

• ET largest fraction, ≈ precip

• Perc very low, elevated only during wettest year

Note: Y2 scale 100x < than Y1

0

200

400

600

800

1000

0

2

4

6

8

10

2001

2002

2003

2004

2005

2006

2007

2008

2009

2010

2011

2012

2013

2014

2015

2016

2017

Precipitation

Evapotranspiration

Percolation

Pre

cip

itation &

Evapotr

anspiration (

mm

)

Perc

ola

tion (m

m)

Cheney: Percolation & Precipitation

• Nearly ALL of precipitation becomes ET

• Perc elevated in very wet year only. Not dependent on magnitude of precipitation.

200

300

400

500

600

0

1

2

3

4

5

200 300 400 500 600

Evapotranspiration

Percolation

Eva

potr

anspira

tio

n (

mm

)

Perc

ola

tion (m

m)

Precipitation (mm)

White Mesa MillTailings FacilityElevation: 1900 mPrecipitation: 360 mmClimate: Seasonal semi-aridCover Thickness: 2.0 m

Clay RadonBarrier

GravelAmended

Water Storage

& Protection

200 mm

1000 mm

910 mm

Tailings

ClayRadonBarrier

Interim

White Mesa Water Balance Record

0

200

400

600

800

0

50

100

150

200

250

300

350

9/20/16 11/21/16 1/23/17 3/26/17 5/28/17 7/29/17 9/30/17

White Mesa, UT

Soil Water Storage

On-Site Precipitation

On-Site Evapotranspiration

Surface RunoffPercolation

NWS Evapotran-

spiration

NWS Precipitation

Lateral Flow

VaultFlooded

So

il W

ate

r S

tora

ge

, C

um

ula

tive

R

un

off

, L

ate

ral F

low

, a

nd P

erc

ola

tio

n (

mm

)

Cu

mula

tive

Pre

cip

itatio

n a

nd

Eva

po

tran

sp

iratio

n (m

m)

• Large seasonal change in soil water storage

• Vegetation being established, allowing longer residence of water in profile

• Perc, lateral flow, and runoff very low. Little erosion potential

Lessons Learned• Water balance covers very effective in managing water

balance at semi-arid sites for broad range of conditions. Percolation typically < than 0.5 mm/y on average.

• Water balance covers consistent with natural ecological setting, promoting long-term sustainable performance with minimal maintenance.

• Armored resistive barrier functions well with modest precipitation, but long-term integrity of the barrier layer critical to proper functioning.

• At higher precipitation, percolation from cover with resistive earthen barriers increases substantially.